Objective: 

Acute application of nucleus pulposus (NP) on the 4th lumbar (L4) dorsal root ganglion (DRG) in a rodent model results in a rapid increase in the number of afferent evoked responses in the contralateral ventrolateral nucleus (VPL) of the thalamus (Brisby & Hammar, 2007). The aim of the study was to test the hypothesis that prior NP exposure sensitizes pain pathways to a subsequent experimentally induced acute disc hernia (mechanical dislocation and repeated NP application).

Methods: 

In 8 adult rats the L4-L5 intervertebral disc was punctured resulting in NP leakage, 7 sham operated animals were used as controls. After 24 hours, the animals were anaesthetised and the L4 DRG mechanically dislocated. Responses in the VPL evoked by electrical stimulation of the sciatic nerve at 20T were recorded for 20 minutes. Following repeated NP application the recordings continued for further 20 minutes.

Results: 

Baseline records were obtained prior to mechanichal dislocation and set at 100%. Mechanical dislocation of the DRG resulted in a similar decrease of the mean number of evoked responses in hernia operated rats (76±12%, mean ±SEM) and sham operated rats (78±19%,). Application of NP onto the DRG while maintaining the disclocation resulted in an increase in the mean number of evoked responses in disc punctured animals (101±10%) but had no effect in sham animals (80±21%).

Conclusions: 

The observed decrease in the number of evoked responses after mechanical dislocation of the DRG suggests that prior exposure to NP does not sensitize transmission in pain pathways to mechanical dislocation. We can however not exclude a sensitization to further NP exposure. Ref. Brisby, H. & Hammar, I. 2007. Spine, 32, 2846–52.